A spandrel is a phenotypic characteristic that evolved as a side effect of a true adaptation. The term is originally from architecture, and the meaning in evolutionary biology is analogous. The biological term spandrel was popularized by Stephen Jay Gould and Richard Lewontin in their influential paper "The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme". In the context of evolution, they introduced the term spandrel as a metaphor for characteristics that are or were originally side effects and not true adaptations to the environment. They are traits which confer no adaptive advantage to an organism, but are 'carried along' by an adaptive trait.

One of their examples was the lengthening of a bone in the hind limb of the Giant Panda, as a result of the lengthening of the corresponding bone in the forelimb. The bone in the forelimb has adapted to perform a function similar to that of an opposable thumb. The change in the hind limb, arguably, confers no adaptive advantage, whereas the change in the forelimb certainly does. The important implication of this idea for Evolutionary Psychology being, that therefore not every trait can be accounted for in terms of adaptive advantage. For instance, Gould puts forward the hypothesis that language itself in humans came about as a spandrel: "Natural selection made the human brain big, but most of our mental properties and potentials may be spandrels - that is, nonadaptive side consequences of building a device with such structural complexity" (S.J.Gould. The Pleasures of Pluralism , p.11) But of course, once a trait confers an adaptive advantage, as arguably most of our "mental properties and potentials" do, it is no longer a spandrel, and thus opens the debate concerning the importance of the concept of spandrels.

Critics such as Dennett argue that the architectural spandrels (pendentives, to be precise) of St Mark's Basilica are not the undesigned gaps between design features that Gould and Lewontin describe, but that they are intentionally designed features themselves; deliberate solutions to an architectural problem where alternative solutions available to the designers included the use of corbels or squinches. The critics also argue that this misidentification of a design feature as an accident is an illustration, in parallel, of Lewontin's and Gould's underestimation of the adaptedness of evolved lifeforms.